A solar cell fundamentally comprises a diode having a pn junction. When solar cells are to be used for generation of electricity in practical use, a plurality of solar cells are serially connected to obtain a voltage which is a sum of voltages of the respective cells. When one of the serially connected cells is shaded, however, it is subjected to a reverse direction voltage which is generated by the other serially connected cells. In that case, the shaded cell will block current flow, thereby reducing the output of the solar cell module to a great extent. Furthermore, when the reverse breakdown voltage of the solar cell is low the solar cell may be damaged or destroyed.
In order to prevent this phenomenon, it is necessary to enhance the reverse breakdown voltage of the solar cell or to connect diodes in parallel and in reverse polarity with the junction of the solar cell so that the total voltage of the series connected solar cells does not exceed the reverse breakdown voltage of a cell.
The enhancement in the reverse direction blocking voltage of the solar cell is realized by lowering the impurity concentration of the base layer of the cell. However, solar cells are generally required to have shallow pn junctions and, in particular, in solar cells used in space, the depth of the pn junction is required to be less than 0.3 to 0.5 microns in order to enhance short wavelength sensitivity. It is experimentally possible to make such a pn junction by diffusing impurities into a base layer having the required impurity concentration and obtain a blocking voltage of several hundred volts, but it is quite difficult to do so in mass production. Furthermore, in a GaAs solar cell, it is difficult to produce a base layer of sufficiently low impurity concentration due to the presence of impurities such as O.sub.2 in the GaAs. A reverse breakdown voltage of several tens of volts, at the most, is obtained. Thus, there is a limitation in the enhancement of the reverse breakdown voltage of a solar cell.
Furthermore, a device including a diode connected in reverse parallel with a solar cell to bypass a reverse direction voltage is effective but requires space for the external diodes. This leads to a costly device due to the diode connection and reduces system reliability due to an increase in the number of parts. Especially in extraterrestrial use, which requires high reliability, these problems are important.
There is another method of producing independent diodes which are connected in reverse polarity and parallel with the respective cells of a solar cell device to obtain the same effects as the insertion of external reverse polarity, parallel connected diodes. In order to include a bypass diode simply, the n(p) layer of the solar cell may be electrically isolated from the n(p) layer of the bypass diode, and two cells (p-n junction) may be mutually isolated on an insulating substrate, producing both the solar cell function and the bypass diode function.
FIGS. 3 to 5 show a solar cell element of the structure disclosed in Japanese Published Patent Application 57-204180. FIG. 3 shows a plan view of such a solar cell and FIGS. 4 and 5 show cross-sections taken along lines a--a' and b--b' of FIG. 3, respectively.
In these Figures, reference numeral 1 designates a semi-insulating substrate, such as GaAs. An n-type GaAs layer 2, a p-type GaAs layer 3, and a p-type AlGaAs layer 4 are successively disposed on the substrate 1 on one side thereof and an n-type GaAs layer 5, a p-type GaAs layer 6, and a p-type AlGaAs layer 7 are successively disposed on the substrate 1 on the other side thereof. The layers from n-type GaAs layer 2 to p-type AlGaAs layer 4 constitute a solar cell and the layers from n-type GaAs layer 5 to p-type AlGaAs layer 7 constitute a blocking diode. Reference numeral 8 designates an insulating film which functions also as a reflection preventing film. The p-type AlGaAs layer 4 and the n-type GaAs layer 5 are connected by a positive side electrode 9, and the n-type GaAs layer 2 and the p-type AlGaAs layer 7 are connected by a negative side electrode 10, respectively. Accordingly, in the usual state of the above-described construction, a negative voltage is generated at the n-type GaAs layer 2 and a positive voltage is generated at the p-type GaAs layer 3. However, a reverse direction diode is produced by the n-type GaAs layer 5 and the p-type GaAs layer 6. This diode is not affected by the solar cell operation. When a reverse direction voltage is applied to the layers 2 and 3, that is, a positive voltage is applied to the n-type GaAs layer 3, and a negative voltage is applied to the p-type GaAs layer 5, a positive voltage will be applied to the p-type GaAs layer 6, forward biasing the diode constituted by the n-type GaAs layer 5 and the p-type GaAs layer 6. Then, only a voltage resulting from the forward direction voltage drop of the diode is applied to the n-type GaAs layer 2 and the p-type GaAs layer 3 of the solar cell. As a result, the solar cell will be protected by the diode from the reverse direction voltage.
In a solar cell having the structure shown in FIGS. 3 to 5, however, since a semi-insulating substrate is used as the substrate 1, it is not possible to produce an electrode opposite the light receiving surface of the solar cell, i.e., at the rear surface of the substrate. On the contrary, it is required to expose a portion of the n-type GaAs layer 2 which is disposed on the insulating substrate at a portion of the light receiving surface and to deposit an n-type electrode 10 thereon. Therefore, in large area solar cells, the internal series resistance of the solar cell is increased by the lateral resistance of the n-type GaAs layer 2, and the conversion efficiency is reduced. Furthermore, in order to expose a portion of the n-type layer 2 at a portion of the light receiving surface, the etching depth has to be strictly controlled when selectively removing the p-type layer 3, for example, by wet etching. However, when the p-type layer 3 and the n-type layer 2 are the same material, strict control of the etching depth is quite difficult. Furthermore, when a plurality of solar cells are connected in series, a large amount of heat is applied in making connections to the front surface electrodes and the external connector may reach a temperature above 500.degree. C. In a GaAs solar cell, the depth of the pn junction at the solar cell is limited to about 0.5 microns as discussed above, and the pn junction may be damaged by heat, thereby adversely affecting operating characteristics or cracking the GaAs. These considerations restrict the conditions of the interconnection process.